Redox Reactions. Reduction Oxidation

What is Redox? Loss Electrons = Oxidation Gain Electrons = Reduction 30/09/99 REDOX stands for REDuction/OXidation Oxidation is often thought of as a combination of a substance with oxygen (rusting, burning) Just like with acid/base definitions the definition of oxidation is expanded Oxidation refers to a loss of electrons Reduction refers to a gain of electrons Oxidation and reduction always occur simultaneously As a mnemonic remember LEO says GER Loss Electrons = Oxidation Gain Electrons = Reduction

What’s the point ? REDOX reactions are important in … 30/09/99 What’s the point ? REDOX reactions are important in … C3H8O + CrO3 + H2SO4  Cr2(SO4)3 + C3H6O + H2O Purifying metals (e.g. Al, Na, Li) Producing gases (e.g. Cl2, O2, H2) Electroplating metals Electrical production (batteries, fuel cells) Protecting metals from corrosion Balancing complex chemical equations Sensors and machines (e.g. pH meter)

RedOx Oxidation: Reduction: Gain of oxygen Loss of electrons Loss of oxygen Gain of electrons Increase in oxidation number Decrease in oxidation number

Balanced symbol equation 4 Experiments: Burning magnesium Copper in silver nitrate solution Chlorine solution and potassium iodide solution Exploding hydrogen Word equation Balanced symbol equation

Oxidised – gains oxygen 2Mg(s) + O2(g)  2MgO(s) Must be a redox! Mg  Mg2+ Oxidised – loss of e- +2e- Put the e- in. O  O2- Reduced – gain of e- +2e-

Cu(s) + 2AgNO3(aq)  Cu(NO3 )2(aq) + 2Ag(s) Complete the half-equations Oxidised? Reduced? Cu  Cu2+ Oxidised – loss of e- +2e- Ag+  Ag Reduced – gain of e- +e-

H2(g) + ½ O2(g)  H2O(g) Covalent! Need a new definition. No H+ or OH-

RedOx Oxidation: Reduction: Gain of oxygen Loss of electrons Loss of oxygen Gain of electrons Increase in oxidation number Decrease in oxidation number

Oxidation Numbers The oxidation number of an atom in an element is zero. E.g. Mg in Mg, O in O2.

Oxidation Numbers F -1 O -2 H +1 Cl Oxidation state of C in CO2? The oxidation numbers of atoms in a compound add up to zero. F -1 O -2 H +1 Cl Oxidation state of C in CO2? ? – 4 = 0 ? = +4 Put the +!

Oxidation Numbers F -1 O -2 H +1 Cl +2 Oxidation state of Mg in MgCl2? The oxidation numbers of atoms in a compound add up to zero. F -1 O -2 H +1 Cl Oxidation state of Mg in MgCl2? +2

Oxidation Numbers F -1 O -2 H +1 Cl -3 Oxidation state of N in NH3? The oxidation numbers of atoms in a compound add up to zero. F -1 O -2 H +1 Cl Oxidation state of N in NH3? -3

Oxidation Numbers F -1 O -2 H +1 Cl Oxidation state of S in SO42-? The oxidation numbers of atoms in an ion add up to the charge on the ion. F -1 O -2 H +1 Cl Oxidation state of S in SO42-? ? – 8 = -2 ? = +6

Oxidation Numbers F -1 O -2 H +1 Cl -2 Oxidation state of S in S2-? The oxidation numbers of atoms in an ion add up to the charge on the ion. F -1 O -2 H +1 Cl Oxidation state of S in S2-? -2

Oxidation Numbers F -1 O -2 H +1 Cl -3 Oxidation state of N in NH4+? The oxidation numbers of atoms in an ion add up to the charge on the ion. F -1 O -2 H +1 Cl Oxidation state of N in NH4+? -3

H2(g) + ½ O2(g)  H2O(g) Covalent! Need a new definition. No H+ or OH-

RedOx Oxidation: Reduction: Gain of oxygen Loss of electrons Loss of oxygen Gain of electrons Increase in oxidation number Decrease in oxidation number

H2(g) + ½ O2(g)  H2O(g) H +1 O -2 Covalent! Need a new definition. Covalent! +1 O -2 Need a new definition. No H+ or OH-

H2(g) + ½ O2(g)  H2O(g) H +1 O -2 Oxidised? Reduced? O – decrease +1 O -2 Oxidised? Reduced? O – decrease in oxidation number H – increase in oxidation number

Oxidation Numbers and names To avoid any confusion when an element can have several oxidation numbers, the oxidation number is usually mentioned in the compound’s name. In names like “elementate(X)”, the number refers to “element” and not the associated oxygens. So if we look at some examples , we get the following names:- KMnO4 potassium manganate(VII) NaClO3 sodium chlorate(V) POCl2F phosphorus(V) oxydichlorofluoride NaH2PO3 sodium dihydrogenphosphate(III) K2Cr2O7 potassium dichromate(VI) Check the numbers.

Mg is the reducing agent LEO says GER : - Losing electrons is oxidation, and the substance that loses the electrons is called the reducing agent. - Gaining electrons is reduction, and the substance that gains the electrons is called the oxidizing agent. Mg(s) + S(s) → MgS(s) Mg is oxidized: loses e-, becomes a Mg2+ ion Mg is the reducing agent S is the oxidizing agent S is reduced: gains e- = S2- ion

OIL RIG

Not All Reactions are Redox Reactions - Reactions in which there has been no change in oxidation number are NOT redox reactions. Examples:

Trends in Oxidation and Reduction Active metals: Lose electrons easily Are easily oxidized Are strong reducing agents Active nonmetals: Gain electrons easily Are easily reduced Are strong oxidizing agents

Well done!